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Wear of ultra-high-molecular weight polyethylene (UHMWPE) in joint implant applications has been shown to increase with cross-shear (CS, i.e., multidirectional sliding) but decrease with higher contact pressure (CP). Moreover, structural changes, resulting in protrusions, are known to occur to the surface of the pin following multidirectional sliding. However, these phenomena are not yet fully understood. In this study, we simultaneously varied CP and CS to derive an empirical formula for the wear factor as a function of these parameters. The wear factor increased when going from unidirectional sliding to multidirectional sliding but decreased with increasing CP, as has been previously observed. Following these tests, the protrusions on the pin surface were chemically and mechanically characterized to gain insights into both their origin and influence on wear behavior. Micro-FT-IR confirmed that the structures consist of polyethylene, rather than adsorbed, denatured proteins. It also allowed the crystallinity of both the protrusions and unaffected UHMWPE to be estimated, showing a strong positive correlation with the hardness of these different areas on the surface. Time-of-flight secondary-ion mass spectrometry was used to probe the chemistry of the surface and near-surface region and indicated the presence of contamination from the test fluid within the structure. This suggests that the protrusions are formed by the folding of UHMWPE following plastic deformation. It is also suggested that the higher hardness of the protrusions affords some protection of the surface, leading to the observed anomalous behavior, whereby wear increases with decreasing CP. [ABSTRACT FROM AUTHOR]

With the increasing market demand for artificial hip joints, total hip joint replacement has gradually become an effective means of treating a series of hip joint diseases. In order to improve the service life of artificial hip joints, some new artificial hip joint materials, including polyetheretherketone (PEEK) and carbon fiber reinforced polyetheretherketone (CFR-PEEK), have been developed. In this paper, pin-on-plate wear tests under different cross-shear ratios and contact pressures were carried out to study the wear mechanism and worn surface topography of PEEK and CFR-PEEK. The experimental results showed that the wear of PEEK was associated with cross-shear, while CFR-PEEK was not. When the cross-shear ratio was 0.039 and contact pressure was 3.18 MPa, PEEK had poor wear resistance and its wear factor was about eight times that of ultra-high molecular weight polyethylene (UHMWPE). The wear resistance of CFR-PEEK had a significant advantage, since its wear factor was about 30% of that of PEEK. The wear factors of PEEK and CFR-PEEK increased as the contact pressure increased. The arithmetic average of the height amplitude of the surface, Sa, also increased gradually according to the topography of the worn surface. The wear mechanisms of PEEK and CFR-PEEK were scratching, plough cutting, and abrasion Since CFR-PEEK had good wear resistance and insensitivity to cross-shear motion, it is suitable for making artificial hip joints under low contact pressure condition. [ABSTRACT FROM AUTHOR]

With the increasing market demand for artificial hip joints, total hip joint replacement has gradually become an effective means of treating a series of hip joint diseases. In order to improve the service life of artificial hip joints, some new artificial hip joint materials, including polyetheretherketone (PEEK) and carbon fiber reinforced polyetheretherketone (CFR-PEEK), have been developed. In this paper, pin-on-plate wear tests under different cross-shear ratios and contact pressures were carried out to study the wear mechanism and worn surface topography of PEEK and CFR-PEEK. The experimental results showed that the wear of PEEK was associated with cross-shear, while CFR-PEEK was not. When the cross-shear ratio was 0.039 and contact pressure was 3.18 MPa, PEEK had poor wear resistance and its wear factor was about eight times that of ultra-high molecular weight polyethylene (UHMWPE). The wear resistance of CFR-PEEK had a significant advantage, since its wear factor was about 30% of that of PEEK. The wear factors of PEEK and CFR-PEEK increased as the contact pressure increased. The arithmetic average of the height amplitude of the surface, Sa, also increased gradually according to the topography of the worn surface. The wear mechanisms of PEEK and CFR-PEEK were scratching, plough cutting, and abrasion Since CFR-PEEK had good wear resistance and insensitivity to cross-shear motion, it is suitable for making artificial hip joints under low contact pressure condition.

Metal‐on‐polyethylene (MoP) is the most commonly used bearing surface in primary total hip arthroplasty (THA). Polyethylene wear debris remains a major concern. Studies investigating the wear performance based on patient‐specific in vivo kinematics and component orientation remains largely lacking. The primary goal of this study was to identify patterns of the distribution of sliding distance and cross‐shear ratio among THA patients. A validated approach combining dual fluoroscopic imaging system and computed‐tomography was utilized to quantify in vivo gait kinematics and component orientations in 48 total hips. The distribution of accumulated sliding distance and cross‐shear ratio over the polyethylene bearing surface was calculated and analyzed using principal component analysis (PCA). Strong patient‐specific variation in sliding distance and cross‐shear ratio was observed. PCA detected two principal components (PCs) of the sliding distance that together contribute to 94.8% of the total variation. PCA detected four PCs that together contribute to 86% of the total variation of the cross‐shear ratio. Regression analysis identified a positive association between cross‐shear magnitude and axial and frontal range of motion (RoM). Increased cup inclination, stem anteversion, and reduced cup anteversion may lead to superiorly distributed high cross‐shear region, potentially accelerating wear. Our study investigated, in vivo sliding distance and cross‐shear pattern using a comprehensive patient‐specific dataset and detected several wear indicators under in vivo conditions. These findings provided useful reference values that may help to assess wear in MoP THA patients. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:3151–3160, 2018. Metal‐on‐polyethylene (MoP) is the most commonly used bearing surface in primary total hip arthroplasty (THA). The distribution of accumulated sliding distance and cross‐shear ratio over the polyethylene bearing surface was calculated in 48 total hips using patient‐specific in vivo kinematics and analyzed using principal component analysis. Several wear indicators under in vivo conditions were detected. Strong patient‐specific variation in sliding distance and cross‐shear ratio was observed. These findings provided useful reference values that may help to assess wear in MoP THA patients. [ABSTRACT FROM AUTHOR]

Background Despite the positive results in total shoulder arthroplasties (TSAs), a higher revision rate is documented compared with total hip and knee replacements. Wear is the possible main cause of TSA failure in the long-term. This study investigated the effect of joint kinematics and the influence of the rotator cuff on the polyethylene wear performance in an anatomic TSA. Methods Lifting a load of 2 kg with an abduction/adduction of 0° to 90° was simulated for 2 × 10 6 cycles as a primary motion using a fully kinematic joint simulator. A combined rotation in anteversion-retroversion of ±5° and ±10° was also simulated. The force in the superior-inferior direction and the axial joint compression were applied under force control based on in vivo data of the shoulder. A soft tissue restraint model was used to simulate an intact and an insufficient rotator cuff. Results The highest wear rate in the intact rotator cuff group was 58.90 ± 1.20 mg/10 6 cycles with a combined rotation of ±10°. When an insufficient rotator cuff was simulated, the highest polyethylene wear rate determined was 79.67 ± 4.18 mg/10 6 cycles. Conclusions This study confirms a high dependency of the polyethylene wear behavior and dimension on the joint kinematics in total shoulder replacement. This can be explained by an increasing cross-shear stress on the polyethylene component. The results obtained indicate that additional combined kinematics are an indispensable part of wear tests on anatomic shoulder replacements. [ABSTRACT FROM AUTHOR]

Polyethylene remains the most popular bearing material for total knee arthroplasty (TKA). Despite its widespread use, wear continue to be one of major factors implicated in revision surgery. Sliding distance, cross-shear, and contact stress are the major factors influencing polyethylene wear. As previous studies have either relied on wear simulations, computational modeling, or in vitro measurements to quantify sliding distance and cross-shear, in vivo subject-specific sliding distance and cross-shear after bi-cruciate retaining (BCR) TKA has not been previously reported. The objective of this study was to quantify the 6°-of-freedom (6DOF) in vivo kinematics, sliding distance, and cross-shear in BCR TKA patients during gait. Twenty-nine unilateral BCR TKA patients performed level walking on a treadmill under dual fluoroscopic imaging system (DFIS) surveillance. Cumulative normalized sliding distances between the lateral and medial compartments did not change significantly (p > 0.05) during the gait cycle. Although the total normalized sliding distance was similar between the lateral and medial compartments, the cross-shear at the lateral compartment differed significantly from that at the medial compartment (p < 0.001). Significant differences in the relative length positions of the peak sliding distance and cross-shear were found between the lateral and medial bearing components. The flexion-extension motion of the reconstructed knee was more associated with the linear displacements (anterior-posterior, R 2  = 0.6; lateral-medial, R 2  = 0.8, proximal-distal, R 2  = 0.7) than the angular displacement (varus-valgus, R 2  = 0.18; internal-external rotation, R 2  = 0.28). Despite some differences in peak sliding distance and cross-shear positons, our results suggest similar articular contact patterns between the lateral and medial compartments in BCR TKA patients during gait. The data could provide insights into understanding the potential wear patterns in BCR TKAs. [ABSTRACT FROM AUTHOR]

One of the main causes of hip prostheses failure is the premature wear of their components. Multi-directional motion or “cross-shear” motion has been identified as one of the most significant factors affecting the wear rate of UHMWPE in total hip joint replacement prostheses. To better evaluate the effect of this cross-shear motion on the tribological behavior of different biomaterials, a new wear testing device has been designed and developed. This new instrument is capable to reproduce the “cross-shear” effect with bidirectional motion on bearing materials and to determine coefficient of friction (COF) between surfaces during testing. To validate the functionality of this new testing platform, alumina balls were articulated against Ti-6Al-4V ELI alloy disks in Ringer’s solution. Four different articulation patterns, all with identical path lengths per cycle, were tested. Gravimetric weight loss was converted to volumetric wear data in order to determine the effects of motion patterns on the wear. Worn surfaces were analyzed by scanning electron microscopy. This scientific approach to quantifying the tribological effects of cross-shear provides fundamental data that are crucial in evaluating potential biomaterials for use in knee and hip joint replacements.

In this study, epoxy based composites are evaluated for their mechanical and bio-tribological properties to find an alternative to UHMWPE liner material for hip joints. Structalit/UHMWPE composites were successfully developed with excellent filler-matrix bonding. Lower friction and wear of the composite (with 24 wt% of UHMWPE) against CoCrMo are attributed to the higher bulk mechanical strength and lower interfacial shear strength. Additionally, lubrication was supported by heterogeneity driven lubricant flow at the composite pin surface. Burnishing, abrasion, and protuberances were main surface damage mechanisms of UHMWPE whereas composite had few micro-cracks on its surface when filler loading was less. • Structalit™/UHMWPE composites has been evaluated for their biotribological properties. • A novel Prosthetic Biotribometer™ is designed for wear measurements. • Specific wear rate of the optimized composite was 1.35 × 10- 6 mm3/Nm, which was 53% less than that of UHMWPE. • Burnishing, abrasion, and protuberances were main surface damage mechanism for UHMWPE. [ABSTRACT FROM AUTHOR]

In the present study, numerical and experimental wear investigations on reverse total shoulder arthroplasties (RTSAs) were combined in order to estimate specific wear coefficients, currently not available in the literature. A wear model previously developed by the authors for metal-on-plastic hip implants was adapted to RTSAs and applied in a double direction: firstly, to evaluate specific wear coefficients for RTSAs from experimental results and secondly, to predict wear distribution. In both cases, the Archard wear law (AR) and the wear law of UHMWPE (PE) were considered, assuming four different k functions. The results indicated that both the wear laws predict higher wear coefficients for RTSA with respect to hip implants, particularly the AR law, with k values higher than twofold the hip ones. Such differences can significantly affect predictive wear model results for RTSA, when non-specific wear coefficients are used. Moreover, the wear maps simulated with the two laws are markedly different, although providing the same wear volume. A higher wear depth (+51%) is obtained with the AR law, located at the dome of the cup, while with the PE law the most worn region is close to the edge. Taking advantage of the linear trend of experimental volume losses, the wear coefficients obtained with the AR law should be valid despite having neglected the geometry update in the model. [ABSTRACT FROM AUTHOR]

Abstract: Wear of ultra-high molecular weight polyethylene is highly dependent on cross-shear, but the fundamental relation between them is not well understood. Over a half-dozen parameters for quantifying cross-shear have been proposed in the literature. The parameters are either cycle-based, defined using a cyclic motion path, or memory-based, defined only by motion history. All of the parameters have never been compared under the same test conditions to see which has the best correlation to wear rate and wear factor. This paper reviews and summarizes the different parameters and compares how well they correlate with wear test results from a multi-direction tribosystem under physiologically realistic, ball-on-flat test conditions. Experimentally, most cross-shear magnitudes correlated well to wear rate, with coefficients of determination ranging from 0.64 to 0.96. All cross-shear ratios correlated strongly to wear factor, with coefficients of determination ranging from 0.95 to 0.98. The memory-based definitions are robustly formulated, phenomenologically grounded, and show promise, but some of the cycle-based parameters had equally strong correlations. These results were not able to distinguish the optimal parameter, but they do reiterate the significant effect that cross-shear has on wear. [Copyright &y& Elsevier]

Abstract: Wear of UHMWPE acetabular components is the most serious threat to the long-term success of hip replacements. Consequently numerical and experimental wear simulations are of great interest. The present study proposes a mathematical wear model and compares the most recent wear laws, based on the so-called cross-shear (CS) effect. Simulations highlighted the variability of wear predictions with the wear factors/laws. Moreover a sensitivity analysis underlined that the kinematic conditions affect volumetric/linear wear more than the loading ones. This study confirms the importance of the CS in wear predictions even if some critical issues are still open, requiring further investigations. [Copyright &y& Elsevier]

Wear of polyethylene (UHMWPE) is dependent on cross-shear. The aim of the present study was: (1) to develop a theoretical description of cross-shear, (2) to experimentally determine the relationship between cross-shear motion and UHMWPE wear using a wheel-on-flat apparatus, and (3) to calculate the work it takes to remove a unit volume of wear for the use in advanced computational models of wear. The theoretical description of cross-shear has been based on the previously reported finding that cross-shear is maximal when movement occurs perpendicular to fibril orientation. Here, cross-shear is described with a double-sinusoidal function that uses the angle between fibril orientation and velocity vector as input, and maximum cross-shear occurs at 90° and 270°. In the experimental part of the study, friction and wear of polyethylene were plotted against increasing sliding velocity vector angles, i.e., increasing cross-shear. It was found that wear intensified with increasing cross-shear, and wear depth could be predicted well using the double-sinusoidal function for cross-shear (r 2=0.983). The friction data were then used to calculate the work to remove a unit particle by integrating the frictional force over the directional sliding distance. Using the wear volumes, determined for both longitudinal and perpendicular motion directions, the work to remove a unit volume of material was q y =8.473×108 J/mm3 and q x =1.321×108 J/mm3, respectively. Hence, 6.4 times more work was necessary to remove a unit wear volume in the direction of principal motion (i.e., along the molecular fibril orientation) than 90° perpendicular to it. In the future, these findings will be implemented in computational models to assess wear. [ABSTRACT FROM AUTHOR]

Polyethylene wear is a great concern in total joint replacement. It is now considered a major limiting factor to the long life of such prostheses. Cross-linking has been introduced to reduce the wear of ultra-high-molecular-weight polyethylene (UHMWPE). Computational models have been used extensively for wear prediction and optimization of artificial knee designs. However, in order to be independent and have general applicability and predictability, computational wear models should be based on inputs from independent experimentally determined wear parameters (wear factors or wear coefficients). The objective of this study was to investigate moderately cross-linked UHMWPE, using a multidirectional pin-on-plate wear test machine, under a wide range of applied nominal contact pressure (from 1 to 11 MPa) and under five different kinematic inputs, varying from a purely linear track to a maximum rotation of ±55°. A computational model, based on a direct simulation of the multidirectional pin-on-plate wear tester, was developed to quantify the degree of cross-shear (CS) of the polyethylene pins articulating against the metallic plates. The moderately cross-linked UHMWPE showed wear factors less than half of that reported in the literature for the conventional UHMWPE, under the same loading and kinematic inputs. In addition, under high applied nominal contact stress, the moderately cross-linked UHMWPE wear showed lower dependence on the degree of CS compared to that under low applied nominal contact stress. The calculated wear coefficients were found to be independent of the applied nominal contact stress, in contrast to the wear factors that were shown to be highly pressure dependent. This study provided independent wear data for inputs into computational models for moderately cross-linked polyethylene and supported the application of wear coefficient–based computational wear models. [ABSTRACT FROM AUTHOR]

Wear testing of polyethylene in total joint replacements is common and required for any new device. Computational wear modelling has obvious utility in this context as it can be conducted with much greater economy than physical testing. Archard's law has become the accepted standard for wear simulation in total joints but it does not account for cross-shear, which is known to increase wear significantly relative to unidirectional sliding. The purpose of this study was to develop a robust cross-shear model applicable to any interface geometry under any kinematic conditions. The proposed metric, x *, is distinguished from existing cross-shear models by the fact that it measures cross-path motion incrementally throughout a motion cycle and quantifies cross-shear based on incremental changes in sliding direction. Validation showed strong support for the predictive capability of x * when applied to pin-on-disc test data. [ABSTRACT FROM AUTHOR]

Abstract: A theoretical model was previously proposed by Wang et al. and Turell et al. to explain the molecular reorientation and wear mechanisms of ultra high molecular weight polyethylene (UHMWPE). This model has been used to explain observed wear behavior in orthopaedic devices using UHMWPE as a bearing surface. Frictional energy exerted into the UHMWPE is dissipated in the form of heat, wear, and also in reorientation of the molecular chains of the polymer. While the previous model addresses the wear portion of this energy, it fails to account for the active molecular reorientation that occurs due to the cumulative stresses experienced by the polymer. Multiple physical tribological studies have shown a discrepancy with the predictions of the original model. By accounting for the molecular reorientation, wear, and orientation softening effects of the total energy exerted into the UHMWPE, the current model improves upon the simplified assumptions of the previous model. This study correctly predicts the cross shear ratio that generates the maximum wear as demonstrated in the physical tribological studies. [Copyright &y& Elsevier]

Abstract: Computational simulations of wear of orthopaedic total joint replacement implants have proven to valuably complement laboratory physical simulators, for pre-clinical estimation of abrasive/adhesive wear propensity. This class of numerical formulations has primarily involved implementation of the Archard/Lancaster relationship, with local wear computed as the product of (finite element) contact stress, sliding speed, and a bearing-couple-dependent wear factor. The present study introduces an augmentation, whereby the influence of interface cross-shearing motion transverse to the prevailing molecular orientation of the polyethylene articular surface is taken into account in assigning the instantaneous local wear factor. The formulation augment is implemented within a widely utilized commercial finite element software environment (ABAQUS). Using a contemporary metal-on-polyethylene total disc replacement (ProDisc-L) as an illustrative implant, physically validated computational results are presented to document the role of cross-shearing effects in alternative laboratory consensus testing protocols. Going forward, this formulation permits systematically accounting for cross-shear effects in parametric computational wear studies of metal-on-polyethylene joint replacements, heretofore a substantial limitation of such analyses. [Copyright &y& Elsevier]

Abstract: A new definition of the experimental wear factor was established and reported as a function of cross-shear motion and contact pressure using a multi-directional pin-on-plate wear testing machine for conventional polyethylene in the present study. An independent computational wear model was developed by incorporating the cross-shear motion and contact pressure-dependent wear factor into the Archard''s law, in additional to load and sliding distance. The computational prediction of wear volume was directly compared with a simulator testing of a polyethylene hip joint with a 28mm diameter. The effect of increasing the femoral head size was subsequently considered and was shown to increase wear, as a result of increased sliding distance and reduced contact pressure. [Copyright &y& Elsevier]

The effect of multi-directional cross-shear (CS) motion and contact pressure on ultra-high molecular weight polyethylene (UHMWPE) wear was investigated in this study, based on an integrated experimental and computational approach. The wear factor as a function of CS was determined experimentally from a multi-directional pin-on-plate wear tester under a nominal contact pressure of 1MPa. A computational wear model was developed which included the effects of CS as well as the load and sliding distance imposed on the hip joint employing a UHMWPE cup against a metallic femoral head under both gait and Leeds ProSim hip joint simulator conditions. The CS ratios were quantified over the articular surface of the UHMWPE cup and the CS-dependent wear factors derived from multi-directional pin-on-plate studies were applied in the computational wear model. Outputs from the computational wear model were validated independently against an experimental hip simulator study. Comparisons of linear and volumetric wear were made between the computational wear model and the hip simulator testing for a nominal conventional (0 MRad) UHMWPE cup of 28mm diameter and a highly cross-linked (10 MRad) UHMWPE cup. The difference between the computed and experimental volumetric wear was approximately 30 per cent for the 0 MRad UHMWPE, although the worn areas between the prediction and the measurement were similar. For the 10 MRad UHMWPE, the discrepancy was reduced to 16 per cent. In both cases, the computational model predicted a lower wear rate than the experimental simulator testing. The effect of using alternative wear factors under a different nominal contact pressure of 3MPa was also considered. The input wear factor to the computational model, derived from a constant loaded pin-on-plate test configuration, may underestimate the dynamic effect due to the variation in the load in the hip joint simulator. [ABSTRACT FROM AUTHOR]

Abstract: A computational model has been developed to quantify the degree of cross-shear of a polyethylene pin articulating against a metallic plate, based on the direct simulation of a multidirectional pin-on-plate wear machine. The principal molecular orientation (PMO) was determined for each polymer site. The frictional work in the direction perpendicular to the PMO was assumed to produce the greatest orientation softening [Wang et al., 1997. Orientation softening in the deformation and wear of ultra-high molecular weight polyethylene. Wear 203–204, 230–241]. The cross-shear ratio (CS) was defined as the frictional work perpendicular to the PMO direction, divided by the total frictional work. Cross-shear on the pin contact surface was location specific, and of continuously changing magnitude because the direction of frictional force continuously changed due to pin rotation. The polymer pin motion was varied from a purely linear track (CS=0) up to a maximum rotation of ±55° (CS=0.254). The relationship between wear factors (K) measured experimentally and theoretically predicted CS was defined using logarithmic functions for both conventional and highly cross-linked ultra-high molecular weight polyethylene (UHMWPE). Cross-shear increased the apparent wear factor for both polyethylenes by more than fivefold compared to unidirectional wear. [Copyright &y& Elsevier]

In the present study, numerical and experimental wear investigations on reverse total shoulder arthroplasties (RTSAs) were combined in order to estimate specific wear coefficients, currently not available in the literature. A wear model previously developed by the authors for metal-on-plastic hip implants was adapted to RTSAs and applied in a double direction: firstly, to evaluate specific wear coefficients for RTSAs from experimental results and secondly, to predict wear distribution. In both cases, the Archard wear law (AR) and the wear law of UHMWPE (PE) were considered, assuming four different k functions. The results indicated that both the wear laws predict higher wear coefficients for RTSA with respect to hip implants, particularly the AR law, with k values higher than twofold the hip ones. Such differences can significantly affect predictive wear model results for RTSA, when non-specific wear coefficients are used. Moreover, the wear maps simulated with the two laws are markedly different, although providing the same wear volume. A higher wear depth (+51%) is obtained with the AR law, located at the dome of the cup, while with the PE law the most worn region is close to the edge. Taking advantage of the linear trend of experimental volume losses, the wear coefficients obtained with the AR law should be valid despite having neglected the geometry update in the model., (Copyright © 2015 Elsevier Ltd. All rights reserved.)